Field of the Invention
This invention relates to a method and apparatus for determining the alignment of motor vehicle wheels. More particularly, this invention relates to a method and apparatus including an opto-electronic image detector for detecting wheel orientation and producing electronic image data representing the wheels, or a reference mounted thereon, computational means for determining the alignment of the wheels, and means for comparing the electronic images, or data corresponding thereto, to previously stored alignment data and generating information which can be used to perform necessary adjustment to the vehicle.
Terms and Definitions
In the vehicle wheel alignment industry the following terms, with corresponding definitions, are commonly used:
Camber is the angle representing the inward or outward tilt from true vertical of the wheel and is positive if the top of the wheel tilts outward.
Caster is the angle representing the forward or rearward tilt from true vertical of the steering axis. When a wheel is viewed from the side, the angle is positive when the upper ball joint (or top of king pin, or upper mount of a McPherson strut) is rearward of the lower ball joint (or bottom of the king pin, or lower mount of a McPherson strut).
Thrust Line (T/L) is a line that bisects the angle formed by the rear toe lines. Lines and angles measured clockwise from the 12:00 axis are positive.
Geometric Center Line, is the line that runs from a point on the rear axle midway between the rear wheels to a point on the front axle midway between the front wheels.
Individual Toe is the angle formed by a front-to-back line through the wheel compared to the geometric center line. Angles pertaining to the left side are positive when clockwise of the thrust line and angles pertaining to the right side are positive when counterclockwise of the thrust line.
Offset is the amount that a front wheel and its corresponding rear wheel are out of line with each other. If there is no offset, the rear wheel is directly behind the front wheel.
Setback is the amount that one wheel on one side of the vehicle is displaced back from its corresponding wheel on the other side of the vehicle.
Steering Axis is a line projected from the upper pivot point of the upper ball joint or top of kingpin, or McPherson strut, through the lower ball joint.
Steering Axis Inclination (SAI) is the angle between the steering axis and true vertical. If the steering axis appears to tilt inward at the bottom of the wheel (as viewed from the driver position), the SAI is positive. SAI also is also known as kingpin inclination (KPI).
Thrust Angle (T/A) is the angle between the thrust line and the geometric center line. Angles measured clockwise from the geometric center line are positive.
Total Toe is the sum of individual, side-by-side toe measurements. If lines projected parallel to the primary planes of the wheels intersect at a point ahead of the side-by-side wheels, the angle is positive (toe in). If the lines would intersect behind the side to side wheels, the angle is negative (toe out). If the projected lines are parallel, the toe is zero.
Traditionally, the Camber and Toe measurements for each wheel of the vehicle are relative measurements i.e. relative to a vertical plane or to another wheel and these measurements are therefore made when the wheels are stationary. On the other hand, the calculation of Caster and SAI is a dynamic procedure and entails determining how the Camber of the front wheels changes with respect to a change in steering angle. This is usually done by swinging the front wheels from left to right through an angle of between 10.degree. and 30.degree., or vice versa, while determining the resultant changes in Camber of the wheel with steering angle changes. From these determinations the Caster and SAI are determined by methods well known in the wheel alignment industry.
Similarly, once Camber, Toe, Caster and SAI have been measured, all other relevant wheel alignment parameters can be calculated by methods and formulations well known in the industry.